I gave it another go with the hint above about the tetherd log:
tail -f ~/.local/state/tether/tetherd.log
I killed that persistent process (sudo killall), rm /run/user/1000/tether/tetherd.* (to clear the lock file), restarted tetherd after pairing to phone via Linux bluetooth settings.
I was then able to see and pair over wifi and bluetooth. Everything is enabled except Notifications (even though both address book sync + notifications are enabled in iOS bluetooth settings).
Clipboard sync working now as well as send/receive files.
The tether app still has occasional issues re-connecting to tether though when I move in and out of the app.
When I start the gtk app, it has a scan devices button but it never finds my phone even though I've paired via bluetooth and its on same LAN.
The iOS app just has a spinning icon that goes forever even though it sees my computer name in the 'discovered devices' list.
I tried pairing from CLI and GTK app and no go on Wifi or Bluetooth.